The operation of modern wireline-logging tools at hostile-environment temperatures places severe demands upon the telemetry as well as the downhole tools. The severe demands on the telemetry are caused by the wide variation in the logging-cable-produced signal distortion combined with the high-data-rate requirements of the tools and the limited logging-cable bandwidth. The large variations in telemetry signal distortion encountered in the course of logging a signal well are the result of the changes in average cable temperature produced by the large difference in borehole temperature between the top and bottom of a hostile-environment well. Telemetry signal distortion is a function of such things as cable manufacturer, cable type, cable length, cable condition, depth in the hole, and well temperature gradient.
The signal distortion produced by the cable must be corrected (equalized) before reliable recovery of the telemetry data at the surface can occur. The high data rate requirement increases the demands on the equalizer in two ways.
First, the high data rate telemetry requirement demands a high bandwidth efficiency (data-rate/frequency-bandwidth) which requires a complex multilevel data-coding scheme. The multilevel coding scheme decreases the voltage difference between different data values thus requiring an increase in the precision of the distortion equalizer to ensure that no data errors occur in the data detection section of the telemetry receiver.
Second, the telemetry signal is more distorted by the logging cable at higher data rates because of the increased interference encountered between the data symbols (encoded bits of data) because of the narrower data symbol time widths.
Another major demand on the signal equalizer is that it be adaptive to compensate for the large variations in telemetry signal distortion encountered in the course of logging the hostile-environment well.
Fortunately, many useful techniques have been developed for digital communication since 1948 when Claude Shannon determined the maximum date rate or information capacity of a coded communication channel. His important contribution was not so much that he presented a way to achieve the theoretical limits, it was more that he showed how poorly the systems of that generation performed. That fact, plus the tremendous need for communication between computers, has motivated researches to develop many new communication techniques, some of which are readily adaptable to building a robust wireline telemetry system. Another critical development has been the rapid increase in performance of low cost DSP (digital signal processing) integrated circuits.
Although many of these developments could be discussed, two techniques found are most useful for building an adaptive high data rate telemetry system capable of logging in hostile environments. Specifically, this disclosure will describe a wireline telemetry system that uses multilevel correlative coding to provide high data rates and adaptive equalization to deal with the variation in channel distortion. In addition, this system makes actual performance measurements of a field system to demonstrate the viability of these new techniques. The benefit of these techniques is that the logging engineer and producing company can be assured of reliable telemetry operation when running a string of modern high data rate wireline tools in hostile environment wells.